1. Field of the Invention
The present invention relates to liquid sampling devices and, more specifically, to a passive diffusion sampler.
2. Description of the Related Art
The efforts to provide for clean water and to prevent contamination of clean water sources require the ability to periodically monitor the quality of surface water and underground water (groundwater) sources for contaminants that may cause health hazards or violate law. Monitoring water quality typically involves obtaining a sample of the water from its place of residence and sending the sample to a laboratory for determination of the chemical make-up, including contaminant type and concentration. It is imperative that the method of sampling allow for a sample that represents the true water chemistry as it would be in-place and minimizes the effect of the sampling technique and the materials.
Frequently, groundwater samples are taken from a small diameter well installed through the soil or rock and intersecting the water that resides between the grains of soil or in the cracks in the rock at a depth of several feet to over a thousand feet. In these cases, the water must be lifted from the well in a manner that ensures the sample chemistry represents that of the intended sampling depth and the sampling method or device does not alter its chemistry. In some cases multiple zones at different vertical levels in the same well or surface water body must be sampled in such a way as to preserve the chemistry of the zone or discrete interval from which it is obtained. In the case of wells, the samples should be obtained with little or no suspended particulate materials, which is evidenced by turbidity. Likewise samples should not be over-exposed to air or be subjected to turbulence as these situations can cause the loss of volatile compounds that should be measured.
Because contamination spreads through water over time and can be conveyed from one location to another, it impacts the use of the land and the potential for liability from harm that may come from exposure to the contaminants. It is therefore necessary to repeatedly check the same locations on a regular and sometimes frequent basis. The cost of the water sampling process and subsequent analysis can have a negative impact on the industry or business deemed the responsible party and ongoing repetitive sampling to meet legal, health requirements, and land use requirements can multiply the cost exponentially.
Some existing systems use manually deployed and recovered “grab” samplers known as bailers or the use of specialized pumps that can provide mechanical lift and greater water volume. Grab samplers are containers, usually cylindrical and closed or with a valve at the bottom end, that are lowered into the fluid on a suspension cord or handle until submerged. The grab sampler fills and then is recovered with the sample inside. Pumps can be of several types, including: submersible pumps, which are lowered into the well and push water upwards, and peristaltic pumps, which sit at the surface with a suction tube into the fluid and which pull the fluid up through the tube and pump to be discharged. To ensure that samples obtained from wells using these methods represent the conditions of the water in the aquifer surrounding the well and not potentially “stagnant” water that resides in the well casing and that is not in contact with the water moving in aquifer itself, the process of “purging” is employed in which larger volume of water than is required for the sample is removed from the well before the sample is obtained. Purging most often involves three, or more, well volumes of water. Purging adds cost to the sampling event because it takes additional time and in some cases the purged water must be containerized and transported to an off-site location for disposal of contaminated materials. The time it takes to purge a well is determined by the capability of the equipment used, the physical characteristics of the well and of the aquifer. The action of purging is influenced by the individual user's techniques and results may vary in the same zone or results may not represent the intended sampling. In some cases, the combination of these several characteristics make well purging impractical or physically impossible. Whatever the device used, the liquid recovered is decanted into a laboratory sample bottle according to specific procedures, is sealed and taken to the lab.
Passive sampling has been employed to enable discrete interval sampling and to reduce the cost of sampling by eliminating well purging. Passive (also referred to as “no-purge sampling”) involves deploying a device into the flow zone of the well (usually the area where the well-screen is located) or in open water. The sampler is left in place for a period of time and then recovered with a representative water sample enclosed. The amount of residence time is determined by the type of sampler and the ability for water of interest to be in contact with the sampler.
In passive diffusion sampling, a single semi-permeable membrane is formed into a container, usually tubular, filled with deionized water and sealed closed. The semi-permeable membrane is selected to allow certain molecules to pass or diffuse through the membrane when the device is placed in a fluid containing those molecules. When a diffusion sampler filled with deionized water is immersed in water having molecules of a type that will diffuse through the membrane, a concentration gradient for that compound will exist between the inside and outside of the membrane. The gradient will cause the molecules in the fluid with higher concentration to pass through the membrane to the fluid with lower concentration until the concentration on each side of the membrane is the same and then the concentration will maintain a dynamic equilibrium with the surrounding fluids. In this way, after sufficient time for diffusion has elapsed, the sample will represent the concentration of the compounds of interest in the fluid into which it is placed, as long as the membrane will allow for diffusion of that compound and the volume of fluid around the sampler is sufficiently large. Groundwater sampling using an appropriate and properly deployed passive diffusion method allows for a sample from a specific interval and screens out most suspended particles that may interfere with lab analysis.
Several types of membranes have been used for single membrane passive groundwater sampling, including:                Polyethylene because the samplers are inexpensive and work well for a range of compounds known as Volatile Organic Compounds (VOCs). Polyethylene membrane diffusion bag (PDB) samplers do not obtain samples of metals, semi-volatile compounds, or volatile compounds that are highly soluble, which together make up a substantial portion of the water sampling need. The method of using polyethylene based membranes for passive sampling is widely used and accepted for those volatile organic compounds that readily diffuse through the membrane.        Regenerated Cellulose Membrane Diffusion Samplers (RCMDS) allow diffusion of a wider range of compounds than Polyethylene membrane samplers. The membrane will allow diffusion of VOCs and some metals and some semi-volatile compounds. An inherent problem with RCMDs is that the cellulose material from which they are made is biodegradable making it susceptible to unpredictable field failure as the membrane is attacked by microorganisms causing it to become weak and then on occasion fail catastrophically, losing its sample contents before they can be used. The RCMD membranes are substantially more expensive than the PDB membranes, which reduces a second key benefit of the passive sampling technique.        Rigid Porous Polyethylene Diffusion samplers (RPPDs) consist of a short length (about 8-inches long) of small diameter porous polyethylene pipe that is filled with deionized water and sealed at each end. The pipe has a much thicker wall than the polyethylene membrane PDB samplers. The pipe has small pores that allow a hydraulic connection between the inside and outside of the sampler. RPPD samplers are typically suitable for some metals and semi-volatile compounds. However, they must be pressurized with deionized water to force the air out of the pores before use, making them inconvenient; they are usually not suitable for VOCs; the pores are typically sufficiently large that water molecules will pass through them under conditions of only a few inches of pressure or water head so that they only hold a small amount of sample without leaking and thus making it necessary to install multiple devices to obtain sufficient sample for most lab analysis; and they tend to be significantly more expensive than PDB or RCMD samplers.        Membrane Channel samplers use permanently installed pipe, sometimes with multiple vertical channels in which each channel is hydraulically connected to a single membrane at a specific depth. A pump for pumping the sample out of the channel is required. Pumps small enough to fit into the channels are not usually capable of recovering adequate sample volume from depth because of physical lift limitations. Additionally, when the water enters the membrane it will rise in the channel or “seek its own level” until the level is equal in the channel to the pressure head at the entry point outside the channel. The liquid in the channel becomes hydraulically isolated from the source and over time may not represent the water quality in the aquifer. And, once pumping begins the method ceases to be truly passive. The method is permanent and only has a single membrane per sample channel. Removing the sample from the chamber at depth can be difficult and may not be feasible in certain environments.        
Hybrid Diffusion/Grab samplers (known by the brand name “Snap Sampler”) operate using cylindrical collection bottles which have an orifice on each end of the cylinder. The device is submerged to the depth at which a sample is to be obtained and left in place for molecules to diffuse throughout the water in and out of the cylindrical bottles. When sufficient time has elapsed spring loaded caps are forced over the two orifice and the contents of the bottles are trapped and recovered. This method is unlike the membrane diffusion method and requires bulky and costly installation cables and triggers that will trigger the caps from the surface. On occasion the caps do not seal properly and/or the sample volume can be less than desirable.
Equilibrated Grab Samplers (known by the brand name “HydraSleeve”) are narrow sleeves of polyethylene sealed at the bottom and with a valve mechanism at the top. The samplers are installed empty as a flat ribbon-like installation. After a suitable residence time for the surrounding well water to recover from the disturbance created by installation, the sampler is pulled at a rapid rate by its suspension cord. During the upward movement the sampler fills with water through the top valve, which seals capturing and isolating the sample within the device. These devices do not use a semi-permeable membrane to acquire the sample via diffusion but do rely on the natural diffusion of molecules through the well and aquifer to maintain representative concentrations in the zone where the sampler is activated. The performance and end result can be negatively affected by the user's actions during installation and activation and the samplers do not restrict silt and colloidal particles from entering the sampler.
Therefore, there is a need for a simple passive sampler that can take a liquid sample that remains undisturbed during the retrieval process.